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ECEN474: (Analog) VLSI Circuit Design Fall spalermo/ecen474/lecture18_ee474_ota_cmfb.pdf · PDF file Sam Palermo Analog & Mixed-Signal Center Texas A&M University ECEN474: (Analog)

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  • Sam Palermo Analog & Mixed-Signal Center

    Texas A&M University

    ECEN474: (Analog) VLSI Circuit Design Fall 2012

    Lecture 18: OTA CMFB Examples

  • Announcements

    • No class on Monday • Preliminary report still due Monday (11/19)

    • If you are doing anything not on the list, give me a brief description as soon as possible

    • We will have class on Wednesday (11/21) • Project Extra Credit

    • Potential for 20% extra credit if you layout a key block • Report on Dec. 4 doesn’t have to have layout • For extra credit, an updated report can be turned in during the

    presentation time (Dec. 10) with layout results • The post-layout performance is the only thing that will be considered

    relative to the original report, i.e. no major circuit changes from the original report

    2

  • Agenda

    • Multi-OTA stages CMFB • OTA-C filter w/ CMFB example

    3

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 4 -

    2 IB

    IB+icmfb

    GND

    VC

    IB

    IB

    IB IB

    GND

    IB

    VC

    CMFB is required for Differential Structures

    CMFB Requirements: Fixes the OTA output (low offset) ==> High dc loop gain Reduction of common-mode noise==> Large Bandwidth

    CMFB CMFB

    Rg1 g

    G m

    m m + =

    R R mm gG =

    GND

    CMFB icmfb = gcmfb(v01+v02-Vref)

    IB+icmfb IB+icmfb

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 5 -

    IB

    VB1

    IB IB

    VREF IB

    Common-mode loop gain = AV Gmp RL 3 poles in the CMFB loop. Loop stability requires AV Gmp / CL < ωp2 @ VC, ωp3 @ VB1

    IB

    IB IB

    IB

    VB2

    VDD

    VSS

    VC

    Efficient CMFB for Differential Pair Based OTAs

    + - AV

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 6 -

    IB IB

    Sensitive to supply noise and common-mode input signals

    IB

    IB IB

    IB

    VDD

    VSS

    V1+v1 V2+v2

    From CMFB

    to next stage

    IB

    GND

    VDD

    VSS

    From CMFB

    to next stage

    v2 v1

    MC

    M1

    R1 R1

    M1

    Pseudo-Differential OTAs with Source Degeneration

    Little sensitive to supply noise and Common-mode noise

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 7 -

    IB

    IB IB

    IB

    CMFB

    IB

    IB IB

    IB

    VDD

    VSS

    IB

    GND

    M1 M1 R1 R1

    M1

    Efficient CMFB for Pseudo-Differential OTAs

    common-mode detector

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 8 -

    OTA based on complementary differential pairs

    -v1 v1 v0 -v0

    VCP

    VCN CLCL

    M1 M1

    M3 M3

    M4 M4

    M2 M2

    VDD

    VSS

     Efficient OTA based on linear complementary differential pairs

     Linear circuit due to source degeneration M3 and M4

     Suitable for fast applications

    11 22 2

    31

    1

    + +

    + =

    Mm

    m

    Mm

    m m Rg

    g Rg gG

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 9 -

    Filter is based on Biquadratic Cells: Biquad Realization in Gm-C topology

    !!! Fast CMFB is required

    vi-

    vi+

    vo-

    vo+ + -

    - +

    + -

    - +

    + -

    - +

    + -

    - +

    gm1 gm2 gm1

    2C2

    2C2

    2C1

    2C1

    gm1

    f0 (MHz) Gm1 (mA/V)

    Gm2 (mA/V)

    Biquad 1 537.6 5.4 9.6 Biquad 2 793.2 5.4 5.07

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 10 -

    Time Domain characterization of the CMFB Common-mode characterization using common-mode current pulses One CMFB circuit per pole •Pulse response of the

    CMFB •Phase margin is better than 45 degrees

    CMFB

    icm

    icm

    vi-

    vi+ vo- vo+

    + -

    - +

    + -

    - +

    + -

    - +

    + -

    - +

    CMFB

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 11 -

    -v1 VREF

    Previous OTA CMFB Next OTA

    v1

    A B

    C

    DE

    v03 v04v01 v02

    OTA with Class AB Common-mode Feedback

    • Common-mode signal is detected at next stage • Class AB error amplifier is used • 5 non-dominant poles at A~E • 2 LHP zeros at A and C (Helpful in BW extension)

    • Most important non-dominant pole at D

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 12 -

    Optimized Class AB Common-mode Feedback

    • Most important non-dominant pole at B

    v03 v04-v1 v1v01 v02 VREF

    Previous OTA CMFB Next OTA

    A A' B

    C

    M1 M3

    M6

    M5 E'

    E

    • Class AB error amplifier is used • 4 non-dominant poles at A~E • 2 LHP zeros at A and C (Helpful in BW extension) • Node D was eliminated

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 13 -

      

       

     +

     

       

     +

     

       

     +

      

       

     +

    1m

    D

    03

    B

    L

    L

    L 6m

    5m

    5m

    VCMFB

    g

    sC 1

    g

    sC 1

    g

    sC 1

    g g

    g2 1

    g

    A

    Analysis of Class AB Common-mode Feedback

    •2 pole-zero pairs (A and C) are very close to each other

    •More stable CMFB

    CMFB can be simplified taking advantage of circuit’s symmetry

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 14 -

    Remarks • DC operating points for high impedances are difficult to fix • Fully differential amplifiers with high output impedance nodes must use common-mode feedback circuits . • Common mode circuits can fix the DC operating points as well as minimize the common mode output components. • Low voltage constraints impose optimal bias conditions at both the input and output ports of an amplifier. • Common mode circuits for LV should be used both at the input and output

  • Next Time

    • Analog Applications • OTA-C Filters • Variable-Gain Amplifiers • Switch-Cap Filters, Broadband Amplifiers

    • Output Stages • Bandgap Reference Circuits • Distortion

    15

    ECEN474: (Analog) VLSI Circuit Design Fall 2012 Announcements Agenda Slide Number 4 Slide Number 5 Slide Number 6 Slide Number 7 Slide Number 8 Slide Number 9 Slide Number 10 Slide Number 11 Slide Number 12 Slide Number 13 Slide Number 14 Next Time

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